1. Field of the Invention
The present invention relates to a solid-state imaging device, such as a CMOS solid-state imaging device. In particular, the present invention relates to a color solid-state imaging device using a color filter.
2. Description of the Related Art
In a solid-state imaging device used in an image sensor or the like, incident light can be collected into a light-receiving portion formed of photodiodes of respective pixels and then photo-electrically converted into signals.
The light-receiving portion formed of photodiodes has sensitivities at the wavelengths of the whole visual light, so that it may not directly obtain color information.
Thus, in the case of a color solid-state imaging device, color filters are installed. The color filters are, for example, those corresponding to the respective colors: Red, Green, and Blue (see, for example, Japanese Utility model Application Publication No. H04-94855; Japanese Utility Model Application Publication No. H04-94529; and Japanese Utility Model Application Publication No. S57-28362).
The most typical color array of a color filter is the Bayer array having four cells in a 2×2 pattern including one red cell, one blue cell, and two green cells.
Another color array is, for example, an array having four cells in a 2×2 pattern divided into four colors, cyan, yellow, magenta, and green using a complementary color filter.
The respective pure colors can be obtained by calculation of signals passing through color filters for the respective colors with a correction algorithm.
In general, a color filter is formed of a specific organic material having a suitable absorption and transmission properties (curve). For example, the color filter may be a pigment-containing resist or the like.
The use of the color filter has a disadvantage in that the color filter may not be a perfect wavelength filter. For instance, rays of light incident on a red cell may not completely become red light rays.
Therefore, correction algorithms have been designed so that more pure colors can be recovered from incident light through color filters for the respective colors. In addition, the overlap of the spectral characteristics of the color filters of the respective colors can be used to facilitate assumptions about pure colors. Therefore, a certain mixing result can be supplied under various conditions in which an image sensor is employed.
On the other hand, Japanese Unexamined Patent Application Publication No. 2004-103964 proposes, instead of using the color filters, a technique for separating colors from light lays using the fact that light rays of different colors differ in depth of absorption in silicon.
In other words, red light tends to permeate more deeply into the silicon than blue light.
This technique may not need any color filter because it can estimate the respective color components even without having any color filter.
However, in this technique, a disadvantage remains in color mixing. For example, when red light and green light pass through a layer that detects blue light, they may be absorbed in the layer to some extent. Therefore, a signal may be generated by the red or green light even when the blue light may not originally be present. As a result, a false signal may occur at a blue pixel.
Japanese Unexamined Patent Application Publication No. 2000-340776 discloses, for example, a structure including a color filter on the surface of photodiodes through an insulated film having the film thickness equivalent to that of a typical interlayer oxide film. When using the structure, however, color filter components in general may be decomposed in a typical semiconductor manufacture process. That is, the typical semiconductor manufacture process uses a high temperature gas having a high reactivity for a plasma deposition method. A typical color filter composes of organic dye material, whose molecules are large in size and decomposed in the typical semiconductor process.
Moreover, a typical color filter has a spectral characteristic in response to a bandpass of organic dye material. Light rays in a narrow wavelength range having a predetermined center wavelength can be transmitted through the filter according to the spectral characteristic. However, when the organic dye material is decomposed, a light transmission characteristic of the color filter including the organic dye material may be changed greatly to make the color filter unsuitable for use.
Further, when using such structure, it is difficult to apply a CMP (Chemical Mechanical Polishing) process used as a planarizing process. This is because the color filter may be broken in a CMP process carried out after forming the color filter. Therefore, a special planarizing process instead of a typical one becomes necessary.
Japanese Unexamined Patent Application Publication No. Hll-027588 discloses a structure including an inactivation layer as a barrier layer formed of a silicon nitride film having the thickness of several hundreds μm above photodiodes so as to improve the response of the device including the photodiodes. According to this structure, moisture and oxygen that infiltrate into the surface of metal wires and photodiodes can be stopped and therefore, degradation of the characteristic of photodiodes due to such moisture and oxygen can be prevented.
However, it may be difficult to form an effective barrier layer formed of a silicon nitride film having a desired thickness on an uneven surface of the Si substrate, and there is a possibility that water molecules, oxygen or hydrogen may leak to substrate at a portion where the thickness of the layer is insufficient. In addition, since the silicon nitride film is formed in a high-temperature deposition process under 300° C. or more so as to obtain a high resistance as the barrier layer, the color filter formed before the silicon nitride film may be melted in the process.